This document discusses human coronaviruses and their effects. It notes that coronaviruses can cause respiratory and intestinal infections in humans with a wide range of clinical manifestations. Some coronaviruses like SARS-CoV and MERS-CoV cause severe pneumonia and acute lung injury, while others like HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU typically only cause mild colds. Lung pathology from SARS-CoV and MERS-CoV infections show diffuse alveolar damage, cellular infiltration, and viral infection of lung cells. Factors that may contribute to severe lung disease include high initial viral loads, infection of lung epithelial cells, and delayed interferon

2. Human coronaviruses
Part (1)
By
Romissaa Aly
Assistant lecturer of Oral Medicine,
Periodontology, Diagnosis and Dental
Radiology (Al-Azhar Univerisity)

3. Coronaviruses belong to the virus family Coronaviridae and are
enveloped, positive-sense RNA viruses.
The coronavirus genome is approximately 31 Kb, making
these viruses the largest known RNA viruses [1, 2].
Coronaviruses infect a variety of host species, including humans
and several other vertebrates.
1

4. These viruses predominantly cause respiratory and
intestinal tract infections and induce a wide range of
clinical manifestations [3, 4].
Coronaviruses infecting the respiratory tract have long been
recognized as significant pathogens in domestic and
companion animals and as the cause of mild and severe
respiratory illness in humans [4, 5].

5. In general, coronaviruses infecting humans can be classified
into low pathogenic hCoVs, which include HCoV-229E, HCoV-
OC43, HCoV-NL63, and HCoV-HKU and highly pathogenic CoVs such
as severe acute respiratory syndrome CoV (SARS- CoV) and Middle East
respiratory syndrome CoV (MERS- CoV) [6, 7].
Low pathogenic hCoV infect upper airways and cause seasonal
mild to moderate cold-like respiratory illnesses in healthy individuals.

6. In contrast, the highly pathogenic hCoVs (pathogenic hCoVor
hCoV hereafter) infect the lower respiratory tract and cause
severe pneumonia, which sometimes leads to fatal acute lung injury
(ALI) and acute respira- tory distress syndrome (ARDS), resulting in
high morbidity and mortality [8–12].
Highly pathogenic hCoVs pose a substantial threat to public
health. During the 2002–2003 epidemic, SARS-CoV infected
approximately 8400 individuals with a 9.6% overall mortality rate
[13, 14].

7. More recently, MERS-CoV crossedpecies to infect 1936
individuals resulting in 690 deaths(∼36% mortality rate) as of
April 5, 2017 [15, 16].
 Recent identification of SARS-like coronaviruses in bats and
MERS-CoV in domesticated camels makes it likely that these
viruses will continue to cross species barriers and cause
additional outbreaks in human populations [17–20].
Thus, there are basically two groups of patients, those
developing milder disease, which resolved and those with severe
disease, which was commonly fatal.

8. The disease severity in pathogenic hCoVinfections was also
influenced by several factors such as initial viral titers in the
airways and age and comorbid conditions of the infected
individual.
While younger individuals below 18 years experience mild-
moderate clinical illness, el- derly individuals exhibit worse
outcomes after infection with SARS-CoV or MERS-CoV [22, 10,
24].
Additionally, individuals with comorbid conditions such as
diabetes, obesity, heart failure, and renal failure among others
experience severe disease, particularly after MERS-CoV infection
[25, 26].
.
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14. Clinical features of highly pathogenic CoV
infection in humans

15. SARS-CoV infection in humans resulted in an acute respiratory illness that
varied from mild febrile illness to ALI and in some cases ARDS and death [27,
10].
The clinical course of SARS presents in three distinct phases. The
initial phase was characterized by robust virus replication accompanied by
fever, cough, and other symptoms, all of which subsided in a few days.
The second clinical phase was associated with high fever, hypoxemia,
and progression to pneumonia-like symptoms, de- spite a progressive decline
in virus titers towards the end of this phase [28].,

16. During the third phase, ∼20% of patients progressed to
ARDS, which often resulted in death [29, 30]. Because of a
progressive decline in virus titers, the third phase is thought to have
resulted from exuberant host inflammatory responses.
The most common clinical manifestations of MERS include
flu-like symptoms such as fever, sore throat, non-productive
cough, myalgia, shortness of breath, and dyspnea, which rapidly
progress to pneumonia [25, 21].
Other atypical presentations include mild respiratory illness
without fever chills, wheezing, and palpitations.

17. MERS-CoV in humans also causes gastrointestinal symptoms
such as abdominal pain, vomiting, and diarrhea.
The majority of MERS patients with dyspnea progress to develop
severe pneumonia and require admission to an intensive care unit
(ICU).
Although most healthy individuals present with mild-moderate
respiratory illness, immunocompromised and individuals with
comorbid conditions experience severe respiratory illness, which
often progressed to ARDS [21].

18. Overall, MERS-CoVcaused severe disease in primary index
cases, immunocompromised individuals and in patients with
comorbid conditions, but secondary cases of household
contacts or healthcare workers weremostly asymptomatic
or showed mild respiratory illness.

19. Lung pathology of hCoV infections
Gross and microscopic pathology of SARS
Gross and microscopic pathology of MERS

20. Gross and microscopic pathology of SARS

21. Typically, analyses of lungs from patients who succumbed to
SARS showed lung consolidation and edema with pleural
effusions, focal hemorrhages, and mucopurulent material in the
tracheobronchial tree.
Diffuse alveolar damage (DAD) was a prominent histological
feature in SARS lungs [31, 32].
Other changes included hyaline membrane formation,
alveolar hem- orrhage, and fibrin exudation in alveolar spaces with
septal and alveolar fibrosis observed during later stages [32, 33

22.  Staining for viral antigen revealed infection of
airway and alveolar epithelial cells, vascular endothelial
cells, and macrophages [31, 32].
 Furthermore, SARS-CoV viral particles and viral genome
were also detected in monocytes and lymphocytes [31].

23. In addition to these changes, histological examination of lungs
from patients who died of SARS revealed extensive cellular
infiltrates in the interstitium and alveoli.
Thesecellular infiltrates included neutrophils and macrophages
with macrophages being the predominant cell type [31, 32].
These results correlated with increased numbers of neutrophils
and monocytes and lower CD4 and CD8 T cell counts in the
peripheral blood samples of patients with fatal SARS [34–36].

24. Gross and microscopic pathology of MERS

25. Despite numerous laboratory-confirmed cases and deaths
due to MERS-CoV infection in several countries, only one
autopsy reportof MERS in humans is available.
Analysis of lung tissue from this patient showed pleural,
pericardial, and abdominal effusions associated with generalized
congestion, edema, and consolidation of lungs [37]. cells,

26. Similar to SARS-CoV infection, DAD was a prominent feature
in the lungs. Additionally, epithelial cell necrosis, sloughing of
bronchiolar epithelium, alveolar edema, and thickening of
alveolar septa were also noted.
 Immunohistochemical examination showed that
MERS-CoV predominantly infected airways and alveolar
epithelial and endothelial cells and macrophages.
The severity of lung lesions correlated with extensive
infiltration of neutrophils and macrophages in the lungs and
higher numbers of these cells in the peripheral blood of MERS
patients [37].

27. Causes of exuberant inflammatory response

28. Despite several years of research studying SARS and MERS
pathogenesis, specific host factors that drive lung pathology
following hCoV infectionsare relatively unknown.
 However, a careful review of the literature related to SARS-CoV
and MERS-CoV pathogenesis in humans and animal models high-
lights several key factors that may play a crucial role in the initiation
and progression of an exacerbated inflammatory responses.
C57BL/6BALB/c
16hrs Post-infection 48hrs Post-infection
SARS-CoV-N
DAPI
Fig. 1 Staining for SARS-CoV-N antigen in lungs of C57BL/6 and
BALB/c mice at 16 and 48 h post-infection
533

29. 1. Rapid virus replication: A notable feature of pathogenic human
coronaviruses such as SARS-CoV and MERS- CoV is that both
viruses replicate to high titers very early after infection both in
vitro and in vivo [38, 98–100, 28].
 This high replication could lead to enhanced cytopathic effects
and production of higher levels of pro- inflammatory
cytokines and chemokines by infected epithelial cells [99, 68, 12].
 These cytokines and chemokines in turn orchestrate massive
infiltration of inflammatory cells into the lungs [38].

30. Studies from hCoV infections in humans and experimental
animals demonstrated a strong correlation between high
SARS-CoV and MERS- CoV titers and disease severity.
2.hCoV infection of airway and/or alveolar epithelial
cells: Studies from animal models, especially mouse
models, provide correlative evidence for differential
disease out- come if the viruses predominantly infect airway
epithelial cells versus both airway and alveolar epithelial
(type I and type II pneumocytes) cells.

31. In B6 and 129 strains, both of which are permissive to
virus replication but resistant to developing clinical
disease, viral antigen is predominantly located in airway
epithelial cells early after infection.
In contrast, in highly susceptible BALB/c mice, virus antigen is
detected in the lung airways and in alveolar type I and II
pneumocytes .
These results suggest a critical role for hCoV-infected type
I and II pneumocytes in mediating lung pathology and host
susceptibility.

32. 3. Delayed IFN responses: As mentioned in previous sec-
tions, both SARS-CoV and MERS-CoV encode multiple
structural and non-structural proteins that antagonize IFN
responses.

33. hCoV reach high titers very early after infection and harbor
multiple proteins that inhibit the IFN response, suggesting
that an early antagonism of the IFN response might delay
or evade the innate immune response.
The delayed IFN signaling further orchestrates IMM
responses and sensitizes T cells to apoptosis
resulting in dysregulated inflammatory response [38].

34. CoV antagonism of IFN responses and disease severity
To counter innate antiviral cytokine responses, SARS-CoV and
MERS-CoV encode several structural and non-structural
proteins (nsps) that antagonize antiviral immune response.
. SARS-CoV encoded nsp1, nsp3-macrodomain,nsp3-
deubiquitinase (DUB), and ORF3b, ORF6, and ORF9b sub-
vert antiviral response by antagonizing IFN and ISG responses
[84–89].

35. While nsp3 impairs IFN responses by unknown mechanism, nsp1
inhibits IFN responses by blocking STAT1 phosphorylation[90,91].
Additionally, structural proteins such as the membrane (M)
and nucleocapsid (N) proteins dampen IFN signaling by
inhibiting TBK1/IKKe and by un- known mechanisms, respectively
[92–95].
 Similarly, MERS- CoV structural proteins M and N and accessory
proteins orf3, orf4a, and orf4b antagonize IFN responses [85, 96,
97].

36. It should be noted that most if not all of these
putative antiviral mechanisms were demonstrated in
transient expression assays and whether they are
actually important in the context of infectious virus
remains to be determined.

37. Structural and non- structural protein antagonism of IFN
responses further amplifies inflammatory responses by
promoting unrestrained virus replication resulting in
increased viral PAMPs that further dampen IFN signaling
and stimulate PRRs to induce an aber rant inflammatory
response.
Lack of IFN signaling also leads to an excessive
accumulation of Ly6C low monocytes and neutrophils.

38. 4.Monocyte-macrophages and neutrophil
accumulation: Both human and animal studies
demonstrate accumulation of inflammatory monocyte-
macrophages and neutro- phils in the lungs following
hCoV infection.
These cells are the predominant source of cytokines and
chemokines associated with hCoV lethal disease
observed both in humans and animal models [38, 32].

40. 1. Epithelial and endothelial cell apoptosis and vascular
leakage: One of the earliest consequences of rapid virus
replication and exuberant pro-inflammatory cytokine/
chemokine responses is lung epithelial and endothelial
cell apoptosis.
 IFN-αβ and IFN-γ induce inflammatory cell infiltration
and cause airway and alveolar epithelial cell apoptosis
via Fas-FasL- or TRAIL-DR5-dependent mechanisms
[101–103].


41. Additionally, TNF released by IMMs also promotes
the apoptosis of both lung epithelial cells and
endothelial cells (unpublished observation).
Apoptosis of epithelial and endothelial cells
compromises lung microvascular and alveolar
epithelial cell barrier resulting in vascular leakage
and alveolaredema ultimate- ly resulting in hypoxia.

42. 2- Suboptimal T cell response: CoV--specific T cells are cru-
cial for virus clearance and limit further damage to host [64,
104].
 Additionally, T cell responses also dampen overactive innate
immune responses [105, 106].
Exuberant inflammatory responses caused by pathogenic
hCoV diminish the T cell response, in the case of SARS- CoV
infection via TNF-mediated T cell apoptosis, thus resulting in
uncontrolled inflammatory response.

43. 3- Accumulation of alternatively activated
macrophages and altered tissue homeostasis: In some
SARS patients with extended duration of disease,
DAD was accompanied by fibrosis of interstitial and
alveolar spaces and hy perplasia of pneumocytes.

44. Similar histological features were noticed in lungs of SARS-
CoV-challenged STAT−/−
mice on B6 and 129 backgrounds.
Lungs from these mice revealed an enhanced perivascular
infiltration of alternatively activated macrophages, neu
trophils, and fibroblasts accompanied by extensive fibrin
deposition and alveolar collapse, features ob- served during
end stage ALI and ARDS in humans [63, 107].


45. Further studies revealed that abrogation of STAT1
signaling, specifically in myeloid cells, resulted in
alternative activation of macrophages [108].

In addition, a delicate balance between host coagulation
and fibrinolysis processes regulates tissue
remodeling and ALI [109].

46. 4- ARDS: Inflammatory mediators play a key role in the
pathogenesis of ARDS, a primary cause of death in pa-
tients infected with SARS-CoV or MERS-CoV [110, 111].

47. Several pro-inflammatory cytokines, including IL- 6,
IL-8, IL-1β, and GM-CSF, reactive oxygen species, and
chemokines such as CCL2, CCL-5, IP-10, and CCL3
contribute to ARDS [48, 112, 113].
Additionally,uncontrolled epithelial cell proliferation
and impaired tissue remodeling during later stages
induce ARDS leading to pulmonary fibrosis and death.

48. Commonly used therapeutics

49. Corticosteroid therapy:
 Corticosteroids are a class of steroidal hormones that exert anti-
inflammatory functions and are generally used to suppress
inflammatory conditions. During the 2003 SARS epidemic,
corticosteroids were the mainstay of immunomodulatory therapy.
The timely administration of corticosteroids often leads to
early improvement in terms of reduced fever, resolution of
radiographic lung infiltrates, and better oxygenation [118–120].


50. However, while some studies showed no beneficial effect,
other demonstrated adverse outcomes following corticosteroid
therapy during SARS-CoV infection in humans.
Early treatment of corticosteroids in SARS patients
enhanced plasma viral load in non-ICU pa- tients, thus
leading to exacerbated disease [118].

51. Overall, these results show that the timing, dosage, and
duration of corticosteroid therapy are critical if this
intervention is to be beneficial in hCoV infections.
In general, corticosteroid ther apy is not recommended
for treatment of hCoV respiratory infections

52. Interferons: Pegylated and non-pegylated interferons have
been under investigation for therapeutic purposes in hCoV-
infected individuals. However, therapeutic use of these agents
produced mixed results both in humans and animal models of
hCoV infections

53. Early administration of IFN was marginally
beneficial in reducing viral load and resulted in
moderate improvement in clinical manifestations.
 In contrast, delayed administration of IFN did not
have any advantage compared to placebo controls.
Similarly, early administration of combination of
IFN and ribavirin modestly ameliorated dis- ease
severity but did not affect mortality [115, 121, 117, 122].

54. Fig. 2 Schematic representation
of protective versus pathogenic
inflammatory responses to
pathogenic hCoV infections

55. Other possible therapeutics

56. IFN-αβ inhibitors and IFN-λ: IFN-αβ restrict virus replica- tion
through induction of ISGs.
However, IFN-αβ can also exacerbate disease by enhancing
recruitment and function of IMMs and other innate immune
cells.
While an early interferon response was protective in SARS-CoV-
infected mice, delayed IFN-αβ signaling dysregulated the anti-SARS-
CoV immune response suggesting that timing of IFN therapy is
critical in determining the disease outcome.

57. Based on these results, the administration
of IFN-αβ receptor blockers or anagonists
should be considered as an option to prevent
exuberant inflammatory responses during later
stages of severe disease, particularly during SARS
[38].

58. In contrast to IFN-αβ, IFN-λ mainly activates epithelial
cells and lacks monocyte- macrophage-mediated pro-
inflammatory activity of IFN-αβ [123].
Additionally, IFN-λ suppresses neutrophil recruitment to the
site of inflammation [124].
Since SARS-CoV and MERS-CoV predominantly infect
AECs and IFN-λ stimulatesantiviral gene in epithelial cells
without over-stimulating the immune system, use of IFN-
λ may be an ideal therapeutic option.

59. Suppression of oxidized phospholipids :Oxidized phospho-
lipids (OxPL) have been shown to promote ALI by increasing lung
macrophage cytokine/chemokine production via TLR4- TRIF
signaling in influenza A virus (IAV)-infected mice [125].
In a recent study, therapeutic administration of the TLR4
antagonist, Eritoran, protected mice from lethal IAV infection
by reducing the levels of OxPL and inflammatory cytokines
and chemokines [126].

60. Despite potent immuno modulatory functions, Eritoran has
no direct antiviral activity, suggesting its use in the
amelioration of inflammatory responses.
Since pathogenic human coronaviruses cause acute lung injury
and promote OxPL production in the lungs [125], strategies to
suppress OxPL either by using Eritoran or other similar
compounds could be of value in dampening hCoV- induced
inflammation.

61. Sphingosine-1-phosphate receptor 1 agonist
therapy: In mice infected with IAV, sphingosine-1-phosphate
receptor 1(S1P1) signaling in endothelial cells was shown to
orchestrate pathogenic inflammatory responses [127].
Targeted S1P1 agonism restrained excessive inflammatory cell
recruitment, suppressed pro-inflammatory cytokines and
chemokines, and reduced IAV induced morbidity and mortality
[127, 128].

62. SARS-CoV infects lung epithelial cells and endothelial cells
in humans and NHPs [29], so that SARS-CoV infection of
endothelial cells may drive S1P1-mediated inflammatory
cytokine/chemokine responses and neutrophil and macrophage
accumulation.
Therefore, S1P1 agonism could be a potential therapeutic
agent in hCoV patients to dampen pathogenic cytokine and
chemokine responses, if a role for an excessive immune response
by these cells is demonstrated.

63. Other immunomodulatory agents Several other immuno-
modulatory agents that could ameliorate inflammatory re
sponses following pathogenic hCoV infections include
cytokine/chemokine inhibitors and danger-associated molecular
pattern (DAMP) antagonists [131].
Studies from animal models show a significant contribution of
TNF to acute lung injury and impaired T cell responses in
SARS-CoV- challenged mice.

64. In vivo neutralization of TNF activity or infection of
mice lacking TNFR provides protection against SARS-
CoV-induced morbidity and mortality [38, 132].
However, it is to be noted that TNF was not detected in the
serum of SARS patients at least during later stages of
infection.

65. Inhibitors of monocyte recruitment and function Studies in
animal models demonstrate pathogenic roles for IMMs during
lethal hCoV infections.
In a mouse model of cardiac inflam- mation, systemic delivery
of optimized lipid nanoparticles containing a CCR2-silencing
short interfering RNA (siRNA) efficiently degraded CCR2 mRNA
and impaired IMM recruitment to sites of inflammation thus
resulting in improved disease outcome [129, 130].

66. Since hCoVs are single-stranded RNA (ssRNA) viruses
and stimulation of IMMs with the TLR7 agonist, R837 (a
synthetic ssRNA mimic), induces strong inflammatory
responses, it is possible that IMM- specific TLR-7
signaling promotes excessive inflammation in
response to hCoV infection.
Thus, a TLR7 antagonist- targeted approach to mitigate
inflammation could prove beneficial.

67. References: